Many technical applications in electronics and display technology use a thin polycrystalline silicon (Si) layer on glass. Such panels are typically used for liquid crystal display (LCD), organic light emitting diode (OLED) and solar cell technology. The standard process to produce such panels is to first deposit amorphous Si layers on glass by chemical vapour deposition (CVD) or sputter processes. Subsequently a polycrystalline film is formed by laser annealing such as excimer laser crystallization (ELC) or sequential lateral solidification (SLS) techniques. An overview of these different common techniques is given e.g. in U.S. Pat No. 7,061,959 which is herewith incorporated by reference.
A technique for conversion of amorphous silicon into polycrystalline silicon is the so called thin beam directional x-tallization (TDX) process. This process uses a pulsed narrow narrowly focused laser line with a width (so called short axis) dimension of about 10 μm and a longitudinal (so called long axis) dimension of about 500 mm which is scanned in the short axis direction in order to melt the thin Si layer having a thickness of 30 to 100 nm.
When applying the ELC, SLS or TDX processes a thin silicon layer on glass is typically melted by an illumination line being emitted by a high energy excimer laser, such as a XeCl excimer laser, and shaped by beam shaping optics which generally perform at least one of the following: 1) changing the shape and/or divergence in one or two directions perpendicular to the direction of beam propagation; 2) homogenizing the intensity at a field and/or pupil plane in one and/or two directions; and/or 3) changing the spatial and/or temporal coherence.
After having shaped the beam with the beam shaping optics the beam usually has a rectangular cross section which upon further propagation scales in size in the long and/or the short axis direction.
The energy density of the laser line on the silicon layer can be homogenous in the long axis direction and lie within a certain process window. The theoretical process window is reduced by effects like long axis uniformity fluctuations, beam position and pointing fluctuations, variations of the Si film thickness and/or a variation of the overall energy reaching the panel. The latter is often induced by variations of beam parameters of the laser that influence the transmission of the optical system. For these reasons the relevant process parameters have to be measured and stabilized.
Usually, the laser energy is stabilized by a feed back loop that measures the energy close to the exit window of the laser. This is the most frequently used laser energy stabilization technique since nearly each commercially available laser is equipped with such a feedback loop. A laser annealing system comprising such a laser energy stabilization system works quite well, however, it does not prevent variations of energy density along the laser line on the silicon layer leading to poor crystal quality when using the TDX process.